While new kernels and other packages are sent down as Fedora upgrades, major GNOME desktop upgrades generally aren't sent down to the wire for existing stable releases. However, with Fedora 21 still being many months out due to the project's restructuring, according to Fedora contributor Ankur Sinha, they are exploring sending GNOME 3.12 into Fedora 20. The Fedora 20 release up to this point is using GNOME 3.10 and GNOME 3.12 isn't even being officially released for another month while right now it's in beta.
For developers and early adopters wishing to test the latest GNOME 3.12 packages built for Fedora 20, there's now F20 GNOME 3.12 on Copr for those wishing to volunteer and test to ensure the stable GNOME 3.12.0 experience will be good on Fedora 20. Ankur Sinha wrote more about this on his personal blog.
GNOME 3.12 on Fedora 20 is exciting to see since generally Fedora always showcases the latest GNOME technologies and with the 3.12 release there's a ton of improvements, but sadly no new Fedora release on the horizon. Among the GNOME 3.12 work is much improved Wayland support, new GTK+ features, redoing the Gedit UI, and a ton of other work throughout the GNOME stack.